1. Field of the Invention
This invention relates to a vibration/noise control system, and more particularly to a vibration/noise control system which actively controls vibrations and noises generated with a periodicity or a quasi-periodicity from a rotating member and the like, to thereby reduce the vibrations and noises.
2. Prior Art
Recently, vibration/active noise control systems have been intensively developed in various fields of industry. These systems are adapted to damp vibrations and noises produced from vibration/noise sources using an adaptive digital filter (hereinafter referred to as "ADF"), to thereby reduce the vibrations and noises.
These conventional vibration/active noise control systems include a vibration/noise control system proposed in Japanese Laid-Open Patent Publication (Kokai) No. 7-271451 by the present assignee, wherein a sine wave signal having a single repetition period is generated depending on the repetition period of vibrations and noises peculiar to component parts of the vibration/noise source, and the sine wave signal and a delayed sine wave signal which is delayed in phase by a predetermined period relative to the former are input to the ADF.
In the proposed vibration/noise control system, a Wiener filter (hereinafter referred to as "the W filter") of a Finite Impulse Response (FIR) type having two taps (filtering order number) is employed as the ADF (first filter means), and a rotation signal from a rotating member is detected in the form of a pulse signal whenever the rotating member rotates through a predetermined very small rotating angle (e.g. 3.6.degree.). More specifically, in the proposed vibration/noise control system, a sine wave signal for one repetition period is generated whenever the rotating member rotates one rotation (360 degrees), and the thus generated sine wave signal and a delayed sine wave signal obtained by delaying the sine wave signal in phase by a predetermined period are input to the first filter means for executing adaptive control, whereby even with the use of the ADF having two taps, the adaptive control can be achieved, enabling a reduction in the time period required for the product-sum operation (convolution) to be carried out.
According to the proposed active noise control system, an identifying sound is generated, which is lower in level by a predetermined amount than the background noise, so that the transfer characteristic of the noise transmission path can be identified while the active control of vibrations and noises is carried out, without the identifying sound being sensed by the passenger(s).
That is, since vehicles, such as automotive vehicles in which vibrations and noises are generated with a periodicity or a quasi-periodicity, are used to travel under various environments over a long time period, the transfer characteristic of the vibration/noise transmission path changes depending on environments under which the vehicle travels. In particular, when vibration/noise control is carried out for a vehicle in which the engine is mounted on a so-called self-expanding engine mount, there can occur a change in the elasticity of rubber members constituting part of the engine mount due to temperature changes, and/or hardening of the rubber members due to aging, which causes a change in the transfer characteristic. Further, the transfer characteristic of vibrations and noises within the vehicle compartment delicately changes depending on various factors, such as the temperature, the humidity, open/closed states of windows of the vehicle, and seating locations of passengers and the number of the passengers. The proposed active noise control system copes with the above-mentioned circumstances by generating an identifying sound which is lower in level by a predetermined amount than the background noise to identify the transfer characteristic of the noise transmission path while the active control of vibrations and noises is carried out, to thereby achieve adaptive control in a manner compensating for a change in the transfer characteristic due to aging, etc. to some degree.
In the proposed active noise control system which generates the identifying sound at a lower level by a predetermined amount than the background noise, the identifying sound is required to have a good S/N ratio to obtain highly accurate identification results. However, if the identifying sound is set to a higher level to increase the S/N ratio, the identifying sound is sensed by the passenger(s), to thereby give an uncomfortable feeling to the passenger(s). That is, since the proposed active noise control system itself generates an identifying sound to identify the transfer characteristic of the vibration/noise transmission path while the active control of vibrations and noises is carried out, there may be an incompatibility between the reduction of the level of the identifying sound to such a level that it is not sensed by the passenger(s) and the achievement of a good S/N ratio. Thus, the proposed active noise control system can achieve only a limited accuracy of identification of the transfer characteristic of the vibration/noise transmission path in response to aging change and environmental change.